OBJECTIVE: The method for measurement of basal metabolic rate (BMR) using indirect calorimetry in adults is well established but is impractical in infants. METHODS: In this prospective study energy expenditure was measured using indirect calorimetry in 14 infants when sleeping and when lying quietly awake. RESULTS: Sleeping metabolic rate (SMR) was lower than energy expenditure (EE) measured in the same infants in a quiet resting state (mean difference [SD]: 297 [162] kJ/d; P < 0.005; 55 [33.4] kJ/kg per day; P < 0.005). The correlation within individuals suggests that these differences are related to the level of arousal. Awake EE, but not SMR, was significantly greater than estimated BMR using the FAO/WHO/UNU predictive equation. CONCLUSIONS: In infants, the level of arousal during measurement of EE can significantly impact on the interpretation of EE results. A standardized method for the measurement of EE in infants using indirect calorimetry is proposed.